It is proposed to display a television picture by using, for example, a liquid crystal.
In FIG. 6, reference numeral 1 designates an input terminal to which a television video signal is supplied. The signal from this input terminal 1 is supplied through switching elements M.sub.1, M.sub.2, . . . M.sub.m, each of which is formed of, for example, an N-channel FET, to lines L.sub.1, L.sub.2, . . . L.sub.m in the vertical (Y axis) direction where m is the number corresponding to the number of picture elements in the horizontal (X axis) direction. Further, there is provided a shift register 2 having m stages. This shift register 2 is supplied with clock signals .PHI..sub.1H, .PHI..sub.2H each having a frequency m times the horizontal frequency. Picture element switching signals .PHI..sub.H1, .PHI..sub.H2, . . . .PHI..sub.Hm, which are derived from the respective output terminals of this shift register 2 and sequentially scanned by the clock signals .PHI..sub.1H, .PHI..sub.2H are supplied to the respective control terminals of the switching elements M.sub.1 to M.sub.m. To the shift register 2, there are supplied a low potential (V.sub.SS) and a high potential (V.sub.DD) and thereby drive pulses of the two potentials are generated.
To the respective lines L.sub.1 to L.sub.m, there are connected one ends of switching elements M.sub.11, M.sub.21, . . . M.sub.n1, M.sub.12, M.sub.22, M.sub.n2, . . . M.sub.1m, M.sub.2m, . . . M.sub.nm, which are each formed of, for example, an N-channel FET, where n is the number corresponding to the number of the horizontal scanning lines. The other ends of these switching elements M.sub.11 to M.sub.nm are respectively connected through liquid crystal cells C.sub.11, C.sub.21, . . . C.sub.nm to a target terminal 3.
Further, there is provided a shift register 4 having n stages. This shift register 4 is supplied with clock signals .PHI..sub.1V and .PHI..sub.2V each having a horizontal frequency. Scanning line switching signals .phi..sub.V1, .phi..sub.V2, . . . .phi..sub.Vn, which are derived from the respective output terminals of this shift register 4 and sequentially scanned by the clock signals .PHI..sub.1V and .PHI..sub.2V, are supplied through gate lines G.sub.1, G.sub.2, . . . G.sub.n in the horizontal (X axis) direction to control terminals of the switching elements M.sub.11 to M.sub.nm at every rows (M.sub.11 to M.sub.1m), (M.sub.21 to M.sub.2m), . . . (M.sub.n1 to M.sub.nm) in the X axis direction, respectively. Also, the shift register 4 is supplied with the potentials V.sub.SS and V.sub.DD similarly to the shift register 2.
That is, in this circuit, to the shift registers 2 and 4, there are supplied the clock signals .PHI..sub.1H, .PHI..sub.2H, .PHI..sub.1V and .phi..sub.2V which are shown in FIGS. 7A and 7B. Then, the shift register 2 generates signals .phi..sub.H1 to .phi..sub.Hm at every picture element period as shown in FIG. 7C, while the shift register 4 generates signals .phi..sub.V1 to .phi..sub.Vn at every one horizontal period as shown in FIG. 7D. Further, to the input terminal 1, there is supplied a signal as shown in FIG. 7E.
When the signals .phi..sub.V1 and .phi.H1 are generated, the switching elements M.sub.1 and M.sub.11 to M.sub.1m are turned on and thereby a current path from the input terminal 1 through M.sub.1, L.sub.1, M.sub.11, C.sub.11 to the target terminal 3 is formed, through which a potential difference between the signal supplied to the input terminal 1 and that at the target terminal 3 is supplied to the liquid crystal cell C.sub.11. As a result, in the capacitive portion of the cell C.sub.11, there is sampled and then held a charge corresponding to a potential difference made by the signal of a first picture element. The optical transmissivity of the liquid crystal is changed in response to this charge amount. The similar operation is sequentially carried out on the following cells C.sub.12 to C.sub.nm Further, when the signal of the next field is supplied, the charge amounts of the respective cells C.sub.11 to C.sub.nm are re-written.
As described above, the optical transmissivities of the liquid crystal cells C.sub.11 to C.sub.nm are changed in response to the respective picture elements of the video signal, and this operation is sequentially repeated to thereby display a television picture.
By the way, when the display is carried out by the liquid crystal, an AC drive is generally adopted so as to improve its reliability and its service life. In the display of, for example, a television picture, a signal, which results from inverting a video signal at every one field or at every one frame, is supplied to the input terminal 1. In other words, to the input terminal 1, there is supplied a signal which is inverted at every one field or at every one frame as shown in FIG. 7E.
By the way, it is requested to display an arbitrary television picture in the form of a still picture by the above mentioned apparatus. In that case, it has been proposed in the prior art that there is provided a memory having, for example, one field or one frame storage capacity, a desired picture is stored in this memory, it is repeatedly read out therefrom, the signal read out is phase-inverted at every field and then fed to the above mentioned input terminal 1. However, the memory having the capacity of one field or one frame itself is very large in size and expensive so that it is difficult to apply it to a standard commercially available apparatus.
On the other hand, it is proposed to display the still picture by utilizing the memory function of the liquid crystal cell C. That is, in a liquid crystal video display drive circuit having a first sample and hold circuit for supplying a video signal having a polarity inverted at every picture to a plurality of picture elements in a time series fashion, this apparatus is a liquid crystal video display drive circuit which comprises inverting means for inverting the video signal and supplying it to the first sample and hold circuit, a second sample and hold circuit for reading the video signal of the plurality of picture elements in a time series fashion, and switching means for switching a video signal from an external terminal or the video signal from the second sample and hold circuit and supplying it to the inverting means.
However, in the case of this apparatus, each time the display of one field is carried out, the picture is displaced by one picture element each in the scanning direction. As a result, the processing such as to reverse the scanning direction at every one field and the like is carried out. In order to switch the scanning direction as set forth above, a circuit of a large scale must be provided and, there remains the state in which the picture is alternately displaced by one picture element at every one field. It is possible that this will give rise to a flicker and so on.
Since the signal of the liquid crystal cell C is derived, this signal is returned again to the liquid crystal cell C and this operation is repeated to thereby carry out the display of the still picture, if a signal transmission characteristic during such period has a distortion, this distorion is accumulated, deteriorating the quality of the picture considerably in a very short time period. To cope therewith, it may be considered to adjust the gain of the inverting means. However, it is impossible to carry out such adjustment perfectly and it is very difficult to carry out the normal display of the still picture during a long time period.
Further, when the signal is derived from the liquid crystal cell C, if a residual charge exists in a stray capacity of the signal line and the like, this causes the signal to be deteriorated so that the display of the still picture can not be carried out over a long time period.